Today's Culture in Tolerance Analysis Blocks Real Benefits

Here's another example of halfway into the article, I could guess who wrote it. Hot Dawg - it's Norm stirring the pot again and making us think!
I was a drafter for 10+ years before being introduced to GD&T in 1990. That was more than 10 years of working alongside engineers who must also have not known or cared about GD&T, let alone stack-up analysis. (When I think of the countless hours of handlettering paragraphs of notes for runout, TIR, etc, and how GD&T symbols would have been so much simpler, I still roll my eyes & shake my head). It's a long in-grained culture that is resistant to change, but the hope is that people comfortable with 3D design will also champion 3D everything else, & they eventually will _become_ the culture.

Drawing attention to the generally poor ability among engineers to use GD&T effectively (if at all) is always timely. Tolerancing errors sometimes become glaring visible even to management, yet the root cause of prototype or production problems is rarely addressed. What is also a huge cost goes unnoticed. Unnecessarily tight tolerances added substantial cost to nearly every design I reviewed; I generally found the offending engineer unable to understand the analysis and unwilling to risk loosening the belt and suspenders.
Interestingly, it was during the transition from 2D to 3D, but prior to effective CAM tool availability, that I noted a dramatic increase in the GD&T knowledge of quality engineers. This was often enforced by the necessity of programming CMMs. In fact, I frequently worked with mere inspectors who had a better working knowledge of GD&T than the resident Mech and Manufacturing Engineers.
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Since few engineers now work in an environment that houses manufacturing, and more design is done in 3D, GD&T knowledge is more critical than ever before. The design might well be manufactured half a world away by a team that would have difficulty providing feedback even if they spoke the same language. Facility among engineers with GD&T is part of an effective DFM strategy. To have quality or manufacturing engineers, or "checkers" clean up a design after the fact is not as efficient as having the design engineer get it right. And knowledge coupled with use of tools that evaluate stack-up is imperative. Ignoring this guarantees that even advanced mechanical engineering will follow manufacturing offshore.

Thanks for support of my "stirring the pot". But really it is not so much to "stir the pot" as my intent is to simply recognize that tolerance analysis contiues to be over looked. What I hope to accomplish is that some of the higher level management and executives that have the obvious objectives to reduce cogs, get to market faster with the best quality and want to reduce physical prototypes to reduce R&D cost and time; go right into production tooling, etc. begin to realize that colorful spreadsheets do nothing to "produce" product. They make colorful design reviews of information and can be very effective in collecting information. But if the information collected is not valid then its garbage in and garbage out. For example, if a CTQ (an ever popular term for Six Sigma) is directly cascaded (another popular term) to a specific measurement (transfer function) between to features of a part or two parts of an assembly and the toerance analysis is invalid or doesn't exist; "What value is the answer on the CTQ scorecard?" What value is there to the design review? The bottom line is margin/profit. Ultimately when product is produced at a cost and sold for what the market will bear, the truth comes out and the scramble is on in a corrective action mode instead of continous improvement.

Pinsan Cheng(Unregistered)07/10/06 05:23 PM

Tolerance analysis is not a solid engineering/science yet

After I read your message, I decided to let you know my experience with tolerance stackup. You are right on talking about the engineering culture and practice. However, I sadly say that tolerance analysis is not a solid engineering/science yet. Take a look at visVSA manual or training course books, there are all the detailed steps how to build a model and get results. THERE ARE NO THEORETICAL BACKGROUND mentioned. If you want to compare VSA with FEA, you can check all major FEA software manuals and you will notice the theoretical manuals there. I don't understand why visVSA does not provide its theory. You can image if I just follow you without understanding what the software is doing behind, I will not and can not go far. In my practice with visVSA, one of difficulties is the judgement of the results. How do I know I got issues in my models. I prefer thinking the issues with my models, instead thinking the bugs in the software. What will happen if we do have a bug !in the software? How do I know? Yes, technical support there. How much time is needed. In industry, how many chances we have to report to management about the "trooubled" results?

You may want to look at CETOL by Sigmetrix. I am currently using this software @ Caterpillar and in the training there is definite talk of the theory behind it. It uses the vector loop approach as opposed to a Monte Carlo simulation approach to solve. There is some definate advantages to this method. There is work being done at universities on this topic as well (Prof. Ken Chase @ BYU for instance) So I'm sure that this field will come up to speed in time. Remember too that FEA is a mature tool (NASTRAN came out in 1968) so I'm sure the look of Tolerance Analysis will change and grow in the coming years.

Having looked at both VisVSA and CETOL one of the things that implementors may want to consider is how your tool stores it's data. Does it have it's own data base? If so, how is the data associated with the production geometry which goes through many changes and then released (solid models). You may find that you will need to figure out how to manage yet another data base then figure out how to tie it to your Solid Model Revisions to reflect real world results.

Norm's points are good ones. Simply put, it's about cost avoidanace. Advanced packaging, Solid Modeling, Tolerance Analysis, CAE, Plant Layout, etc are all cost avoidance tools and is something which is difficult for most to understand.

First, on the issue of no theoretical VisVSA background material - keep in mind the following. VSA was a company purchased by Engineering Animation Inc. prior to EAI being purchased by UGS. VSA was primarily a consulting company that wrote software for the use of its consultants. VSA was never serious about the sale of software per se. As such, not only was there never any motive to document its software, anything that imparted knowledge to the users took consulting business away, so it was in VSA's best interests for this to remain a "mystery". It is Monte Carlo simulation based, which is about all anyone knows about it. Its subsequent owners had an appreciation for GD&T about equal to the average engineer, so its no surprise that information about it is limited.

Second, on the issue of the vector loop vs. Monte Carlo simulation. Vector loop methods are in general very fast, and the do not require actual shop floor results data to be effective. The tradeoff for this is that vector loop performance deteriorates rapidly as the size of the problem increases, and it does not take into account variability that is introduced as a result of variations due to manufactuing methods and machines. In all cases, IMHO a Monte-Carlo based simulation approach yields superior results, and can do so on very large problems (i.e a complicated assembly with corresponding tooling), but to be most effective requires results from shop floor operations to model the variation upon which Monte Carlo simulation depends.

I can help you with understanding CETOL data strategy and discuss the new CETOL v8.0 architectural enhancements that will make data management and CETOL model re-use a much easier process with the CAD system.
CETOL v8.0 will be released soon, and has extended our Exact Constraint Technology, our solvers and analysis solutions that will allow options for synchronization with the CAD model, storing outside the CAD model, and re-using the CETOL model as a template to be connected to different geometry of similiar design. CETOL maintains a unique identifier with the surface ID on the geometry, which a persistent relationship that allow for the updates you are referring to. You can contact Sigmetrix directly through our website at http://www.sigmetrix.com